1. Field of the Invention
The present invention relates to a power converting apparatus comprising a single or a plurality of selfcommutated power converters (hereafter referred to as "self-commutated converters") in common with a DC circuit, using a self turn-off type semiconductor device (hereafter referred to as "self turn-off device"). More particularly, the present invention pertains to a power converting apparatus which can suppress a fault current which occurs when DC short circuit occurs in any self-commutated converter.
2. Description of the Related Art
There are various methods to interrupting the current caused by when a DC short circuit occurs in a self-commutated converter.
The first method is to use fuses. According to this method, a fuse is inserted in series with each self turn-off device into the circuit of the self-commutated converter comprising multiple self turn-off devices to thereby cut off the fault current by the fuse when melted.
As the second method, the self turn-off function (current interrupting function) of the self turn-off device is utilized to cut off the fault current.
FIG. 6 illustrates the arrangement of the self-commutated converter using a gate turn off thyristor (GTO) as its self turn-off device.
A self-commutated converter 10 comprises GTOs 11V to 11Z and diodes 12U to 12Z which are connected in parallel in the reverse direction to the respective GTOs. The DC terminal of the self-commutated converter 10 is connected to a DC circuit 20, while the AC terminal is connected to an AC system 50 through a transformer 40.
Suppose that the GTO 11X is short-circuited due to some reason while the GTO 11U is conductive. Then, an excessive fault current flows to the GTOs 11U and 11X to cause a DC short circuit. A protection device (not shown) detects the fault current which flows through the GTOs 11U and 11X, and provides a gate disable signal to the GTO 11U which is normally activated. As a result, the fault current which tends to flow between the GTOs 11U and 11X is cut off by the GTO 11U, thus protecting the self-commutated converter 10.
The first method is available for a low-voltage circuit involving a DC voltage of 1 kV or lower; however, when the circuit voltage rises above the level, this method is not applicable because of no fuses available for such a high voltage.
In the second method, the current-increase factor di/dt of the fault current is very large. When the protection device detects the fault current, then provides the gate disable signal to the GTO, therefore, the value of the current flowing in the GT is already beyond the value where the GTO can be rendered off. If the GTO is forced to be turned off under such conditions, the GTO itself may be damaged, simplifying the fault.